1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a rake reception apparatus and method capable of reducing power consumption of a terminal in a Discontinuous Reception (DRX) mode of a mobile communication system.
2. Description of the Related Art
Mobile communication systems have evolved from the early voice-oriented service into high-speed, high-quality wireless data packet communication systems for providing data and multimedia services. In particular, a Universal Mobile Telecommunication Service (UMTS) system, the 3rd generation (3G) mobile communication system that is based on Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS), which are European mobile communication systems and use Wideband Code Division Multiple Access (WCDMA), provides consistent services in which mobile phone or computer users can transmit packet switched text, digitalized voice or video, and multimedia data at a high rate of 2 Mbps or higher, regardless of their location.
A mobile terminal receiving communication services from the mobile communication system is characterized by mobility and portability. To maintain the mobility and portability of the mobile terminal, a rechargeable battery is used as the power supply. For such a mobile terminal, research is being conducted on methods capable of increasing a waiting time of the mobile terminal.
The major power consumption of the mobile terminal includes power consumption by, for example, a sleep current, a digital modem, a Radio Frequency (RF) Part and a Central Processing Unit (CPU). That is, to increase the waiting time of the mobile terminal, the power consumption in each of the elements should be minimized. The sleep current refers to a current that causes power consumption during a period in which the terminal receives no message. For example, the sleep current is consumed by an oscillator, a Liquid Crystal Display (LCD), a microprocessor and a power supply. Among the elements, the oscillator consumes the greatest amount of the sleep current, causing an increase in the power consumption. Therefore, the power consumption of the high-frequency oscillator should be reduced to decrease the total power consumption of the mobile terminal. In addition, to reduce the power consumption by the RF part, the time for which the RF part is turned on should be minimized.
The mobile communication system uses a DRX mode to increase the waiting time of the terminal. A DRX mode terminal wakes up from a sleep state at the position of a paging channel, provides power to a digital modem and an RF processor and performs demodulation on the paging channel. However, the DRX mode terminal returns to the sleep state if there is no paging information.
For example, in the WCDMA system, to reduce power consumption of the terminal, a base station transmits a so-called Paging Indicator (PI) signal, where the PI indicates presence/absence of a Paging CHannel (PCH) including a paging message.
FIG. 1 illustrates a timing diagram of a PI and a PCH in a conventional mobile communication system.
A terminal, while in the sleep state, demodulates a PI 110 before directly demodulating a PCH 120, to determine whether the PCH 120 is transmitted to the terminal itself, and demodulates the PCH 120 only when necessary, i.e. when the PCH 120 is transmitted thereto. Because a length of the PI 110 is much shorter than a length of the PCH 120, it is possible to minimize the time for which the terminal wakes up from the sleep state.
In the mobile communication system, the PI 110 is transmitted over a Paging Indicator CHannel (PICH). In addition, a Common PIlot CHannel (CPICH) is always transmitted over a downlink, and is used as a phase criterion for demodulation of the PICH. The CPICH and the PICH are spread with the same scrambling codes, and are multiplied by different channelization codes for their identification. Herein, because the CPICH is always multiplied by a channelization code #0, i.e. ‘1’, a receiver has no need to separately multiply a received CPICH by a channelization code.
FIG. 2 illustrates an operation of a conventional DRX mode terminal.
A DRX mode terminal wakes up from a sleep state only at the time the PI 110 of FIG. 1 is transmitted, and then monitors the PI 110. However, because there is a high possibility that a position of a multi-path signal has changed during the sleep state, if RF power is turned on the terminal first performs a multi-path search operation 210 in a searcher in advance of detection of the PI 110 as described in FIG. 2. After the multi-path search operation, the terminal performs a finger allocation process 220 in a controller, and a PI demodulation process 230. If the RF power is turned off, the terminal performs a PCH demodulation operation 240. A detailed description of the foregoing operations will be made with reference to FIGS. 3 to 6.
In order to increase the waiting time of the terminal, it is preferable to minimize the time for which the terminal wakes up from the sleep state. In particular, because the power consumed in the RF processor greatly affects the waiting time of the terminal, there is a need to determine presence/absence of the PCH 120 through detection of the PI 110 as quickly as possible, and then turn off power of the RF processor.
In the mobile communication system, the terminal uses a rake receiver to demodulate a received signal in the multi-path environment shown in FIG. 2.
FIG. 3 illustrates a block diagram of a reception apparatus with a rake receiver in a mobile communication system. The reception apparatus includes an RF processor 310 and a rake receiver 320. The rake receiver 320 controls demodulating an RF-processed multi-path signal received from the RF processor 310. The rake receiver 320 roughly includes a searcher 321, a plurality of fingers 325-1˜325-N, a combiner 327 and a controller 323.
The searcher 321 searches for multi-path signals before the PI 110 is received, and the controller 323 allocates the searched one or multiple multi-path signals to the fingers 325-1˜325-N, respectively.
FIG. 4 illustrates a detailed block diagram of the searcher 321 shown in FIG. 3. Referring to FIG. 4, the searcher 321 includes a scrambling code generator 321a, a descrambler 321b, an accumulator 321c, an energy calculator 321d and a detector 321e. The scrambling code generator 321a generates a local scrambling code that is equal to the scrambling code used in a base station. The descrambler 321b correlates a received signal to the local scrambling code. In addition, the descrambler 321b descrambles scrambling codes having different phases with the received signal to simultaneously check several hypotheses. A phase difference of each hypothesis, i.e. a size of each hypothesis being checked, has a regular interval as shown in FIG. 5.
FIG. 5 illustrates a hypothesis check process by a searcher.
The descrambler 321b checks several hypotheses by changing an offset of a scrambling code. Herein, the offset of the scrambling code is referred to as a ‘hypothesis’. The accumulator 321c accumulates output values of the descrambler 321b for a time corresponding to a specific length.
The energy calculator 321d calculates energy of a received signal using the value output from the accumulator 321c. The detector 321e detects several upper multi-path signals from the energies of received signals, output from the energy calculator 321d, i.e. from the energies of several hypotheses, and reports the detection result to the controller 323.
Referring back to FIG. 3, each finger 325 is allocated a position of a multi-path signal from the controller 323, performs demodulation on the multi-path signal, and delivers the demodulation result to the combiner 327. The combiner 327 combines the demodulated multi-path signals provided from the fingers 325-1˜325-N, thereby increasing demodulation performance of the rake receiver.
FIG. 6 illustrates a detailed structure of each finger in the rake receiver.
Referring to FIG. 6, the finger 325 includes a scrambling code generator 325a, a descrambler 325b, a channel estimator 325c, a channelization code generator 325d, a multiplier 325e, an accumulator 325f and a channel compensator 325g. The scrambling code generator 325a generates a local scrambling code that is equal to the scrambling code used in the base station. The channelization code generator 325d generates a channelization code. The descrambler 325b descrambles the scrambling codes having different phases with the multi-path signal allocated by the controller 323. The channel estimator 325c estimates the current channel status for N multiple paths using the output value of the descrambler 325b, and outputs the channel-estimated value to the channel compensator 325g. At the same time, the multiplier 325e multiplies the output value of the descrambler 325b by the channelization code generated by the channelization code generator 325d. The accumulator 325f accumulates the output value of the multiplier 325e for a time corresponding to a specific length. The channel compensator 325g conjugates the channel estimation result from the channel estimator 325c, and complex-multiplies the conjugation result by the value accumulated in the accumulator 325f, thereby performing channel compensation. The channel compensator 325g outputs the channel-compensated value to the combiner 327.
A description will now be made of an operation of a terminal with a rake receiver in a DRX mode.
The searcher 321 searches for multi-path signals before the PI 110 is received, and the controller 323 allocates the detected one or multiple multi-path signals to the fingers 325-1˜325-N, respectively. The fingers 325-1˜325-N each descramble the multi-path signal with a scrambling code, separate a CPICH and a PICH using a channelization code, perform channel estimation using the CPICH and demodulate the PI 110. The demodulation results on the PI 110, output from the fingers 325-1˜325-N, are delivered to the combiner 327, which combines the PI demodulation results and reports the combination result to the controller 323. The controller 323 determines from the demodulation results on the PI 110 whether there is a need to receive the PCH 120, and if not, turns off power of a digital modem and an RF part, thereby transitioning the terminal back to the sleep state.
However, the method of receiving the PI using the fingers needs to provide power to the entire rake receiver, causing an increase in the power consumption.
There is an alternative PI detection of using off-line multi-path search and off-line PI detection to reduce the power consumption. This is disclosed in U.S. Pat. No. 6,748,010 to Butler et al. and U.S. Pat. No. 6,829,485 to Abrishamkar et al. In this method, a terminal wakes up from the sleep state, turns on an RF part, stores a received signal near a PI in a buffer, turns off RF power, searches for a multi-path using the stored received signal in the RF power-off state and off-line detects the PI for the detected multi-path. Here, a correlator of a multi-path searcher is used again as a PI detection circuit.
However, in the WCDMA system, because an interval between the PI 110 and the PCH 120 is short as shown in FIG. 1, if the presence of the PCH 120 is found after the PI 110 is off-line detected, there is not enough time to turn back on the RF part and then receive the PCH 120.